Method for estimating received power and a receiver

ABSTRACT

A receiver, and a method for estimating received power in a cellular radio system having in each cell at least one base station communicating with mobile stations within its coverage area. The mobile stations measure strength of the signal received from a base station, and report the measurement results to that base station. To improve power adjustment, a model describing the dynamic behavior of the signal is created for the received power on each connection. The model is utilized for power adjustment, as well as for making handover decisions.

BACKGROUND OF THE INVENTION

The invention relates to a method for estimating received power in acellular radio system comprising in each cell at least one base stationcommunicating with mobile stations within its coverage area, in whichsystem mobile stations measure strength of the signal received from abase station, and report the measurement results to the respective basestation equipment, and each the base station measures strength of thesignals received from the respective mobile stations.

It is typical of a cellular radio environment that conditions for radiowave propagation change constantly. This is due to changes in thelocation of mobile stations with respect to the base station, as well aschanges in the environment of mobile stations. In the connection betweena mobile station and a base station, the attenuation to which radiowaves are subjected on the radio path thus varies constantly.Consequently, especially the transmission power used by the mobilestation must be monitored continuously, and adjusted at each moment oftime. Generally, the aim is to minimize the transmission power used bythe mobile station, so that both power consumption of the station andinterference caused by the station to other connections will be minimal.Power adjustment is particularly critical in CDMA systems, in which theaim is that each base station receive the transmission of the mobilestations within its coverage area by using the same power level whenpossible.

As attenuation on the radio path between a base station and eachrespective mobile station varies, there must constantly be a possibilityto hand over the communication to another base station as rapidly aspossible, before the connection to the old base station is broken.

The speed and accuracy of both power adjustment and handover depend onthe quality control of the connection between the respective mobilestation and the respective base station. In practice, this has beencarried out in such a way that both the respective base station and therespective mobile station measure the power level of the receivedsignal. The mobile station reports the measurement results to the basestation, which transmits power adjustment commands to the mobilestation, and, as the signal weakens, makes the handover decision.

The power level of the received signal is a continuously changingvariable, and instantaneous measurement results contain some inaccuracy,such as measurement noise and time-dependent error caused by the actualmeasurement devices. Therefore, the measurement results are not usefulas such, but they must be processed in some way to minimize the effectof measurement errors.

In prior art systems, measurement results of received power arefiltered, i.e. averaged, within a time window of a certain size. Whenthis procedure is applied, the worst error peaks can be eliminated fromthe measurement results. The prior art method has the advantage of beingsimple to implement, but it has the drawback of being unadaptable to thebehavior of rapidly changing data. A way to improve the prior art methodis to adjust the width of the time window, but this approach is alsounable to model a changing signal sufficiently well, and thus is notuseful in predicting the future behavior of the signal.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to utilize the measurementresults better than heretofore, by using statistical methods inprocessing the measurement results. In particular, these methods allowconsiderably better prediction of the behavior of the signal power levelthan heretofore, thus allowing remarkably more rapid reactions tochanges in the quality of the connection. Therefore, power adjustmentmay be carried out more accurately than heretofore, and abrupt handoversituations may be responded to more rapidly. In addition to prediction,the methods enable more accurate estimates of the reliability of themeasurement results, as well as estimates of possibly missingmeasurement values.

This is achieved with a method of the type set forth in the foregoingBACKGROUND section, which is characterized in that with the aid ofreceived measurement results, a model describing the dynamic behavior ofthe signal, is created for the received power on each connection, andthat when the model is formed, at least one signal interfering with theconnection is taken into account, and that the model is utilized forpower adjustment as well as for making handover decisions.

The invention also relates to a receiver, for use in a cellular radiosystem comprising in each cell at least one base station communicatingwith mobile stations within its coverage area, in which system mobilestations measure strength of the signal received from a respective basestation, and report the measurement values to the respective basestation equipment, and in which the respective base station measuresstrength of the signal received from mobile stations, the equipmentcomprising means for recording measurement results obtained from mobilestations. The receiver is characterized by comprising means for formingwith the aid of the received measurement results, a model describing thedynamic behavior of the signal for the received power on eachconnection, taking into account at least one signal interfering with theconnection, and means for utilizing the predicted data for poweradjustment, as well as for making handover decisions.

The solution in accordance with the present invention enablesimprovement of power adjustment especially in a microcell environment instreet corner situations. In such a case, a mobile station approachingfrom an intersecting street must rapidly adjust its signal strength inaccordance with instructions from a new base station. Rapid poweradjustment enabled by the invention may significantly improve theoperation of power adjustment in a situation of this kind.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail withreference to the examples in accordance with the accompanying drawings,in which:

FIG. 1 shows a part of a cellular radio system, in which the method ofthe invention can be applied,

FIG. 2 illustrates operation of the method of the invention in a poweradjustment situation compared with prior art methods,

FIG. 3 illustrates a street corner situation, in which rapid poweradjustment is needed, and

FIG. 4 illustrates the structure of a receiver of the invention.

DETAILED DESCRIPTION

In the method of the invention, statistical methods are thus employedfor processing the measurement results of a radio channel. Thus, it ispossible to draw more conclusions on the future changes of the qualityof the radio channel from the measurement results than heretofore, inwhich case it is also possible to estimate missing measurement values.

A model describing dynamic behavior of the signal is formed from themeasurement results of the received signal. In the following, processingof the measurement results is illustrated with Kalman filtering, butother statistical methods may also be used for processing themeasurement results in the method of the invention. Kalman filtering isan optimal time-domain prediction method for linear models. The accuracyof linear models is sufficient for short-term predictions. A so-calledstate space model is formed in connection with Kalman filtering on thebasis of the measurement results. The equations of the state space modelmay be expressed as the observation equation (1):

    y.sub.t =A.sub.t x.sub.t +v.sub.t                          (1)

in which y represents an observation vector, A is a matrix determininghow an unobserved state vector x can be converted to an observationvector y, and v represents an observation noise vector. Statetransitions are denoted by the equation

    X.sub.t =Bx.sub.t-1 +w.sub.t,                              (2)

in which B is a transition matrix and w a noise vector. In bothequations t=1,2, . . . , T. Noise vectors v and w can be assumed to bemutually independent, and their default values can be set to zero.

Let us suppose that signal behavior can be modelled autoregressively inaccordance with the equation:

    x.sub.t =a.sub.1 x.sub.t-1 +a.sub.2 x.sub.t-2 +w.sub.t     (3)

    y.sub.t =x.sub.t +v.sub.t,

which can also be expressed as ##EQU1## in which the matrices occuringin the general formulas can be more easily identified.

In the following, the method of the invention will be set out inconnection with a base station; however, the invention is not limited toit. The method can naturally also be applied in the receiver of a mobilestation.

FIG. 1 thus illustrates a cellular network system, in which the methodof the invention can be applied. The figure shows two base stations,BTS1 and BTS2, each base station serving its own coverage area. The basestations are connected to a base station controller BSC via digitaltransmission links 10. In a situation shown in FIG. 1, a mobile stationMS is located in the area between two base stations, in which the mobilestation is able to receive signals 11, 12 from both base stations.

The mobile station measures the strength of the signal received from arespective base station, and reports the measurement results to thatbase station. Let the measurement results transmitted by the mobilestation be denoted by y(t), in which t=1, 2, . . . T. The respectivebase station controller analyses the measurement results from the mobilestation. Kalman filter estimators now have the form

    x(t|t-1)=Bx(t-1|t-1)                     (5)

    x(t|t)=x(t|t-1)+K(t) y(t)-A(t)x(t|t-1)!,

in which the transition matrix B is determined in each cell on the basisof the dynamic process of the channel. Cells of various kinds can bemodelled by using the above described matrix. The matrix can model,e.g., exponential fading of the signal as a function of distance. Thedimension of the matrix determines the order of the state space model.In the above example (formulas 3 and 4), the order is two. In practice,the model can be determined by examining the measurement results. Knownlink parameters, such as Doppler spread can be utilized in the selectionof the model. When Doppler spread is small, it is known that the mobilestation is moving slowly, which is often the case in an urban area, andthe dimension of the matrix can thus be selected to be small. The gainmatrix K(t) can be calculated recursively based on a prediction errorcovariance matrix.

The one step prediction x(t|t-1) has been calculated directly on thebasis of the previous predicted value, whereas the best estimatorx(t|t), involving the current data value is a weighted average of theone step prediction x(t|t-1) and of the error that occurs in predictingy(t).

The base station is capable of optimal adjustment of the transmissionpower of the mobile station by estimating measured values. The inventioncan be applied specifically in a CDMA cellular radio network, in whichthe base station must, in order to maximize the capacity of the cell,adjust the transmission power of mobile stations so that it receives allstations with the same signal strength. In this case, the accurate andrapid power adjustment enabled by the method of the invention isparticularly advantageous.

The invention can also be advantageously applied in cellular radiosystems in which interference within the same channel occurs. Forinstance, if frequency re-use in TDMA systems is increased in order toachieve a higher frequency efficiency, interference within the samefrequency channel may occur.

FIG. 2 illustrates the accuracy of power adjustment based on estimatescalculated from the measurement results, compared with prior artmethods. In FIG. 2, the horizontal axis represents time and the verticalaxis represents power value. In the figure, a value 21 obtained with aconventional non-predictive method, as well as signal estimates 22 and23 obtained with two different estimation methods have been drawn on theactual signal 20. As can be noticed, the predictive one-step estimate 22provides a more accurate value of the signal than the non-predictivemethod 21. An even more accurate estimate of the signal can be obtainedwith the linear estimate 23. The situation shown in FIG. 2 has beensimulated by using the frequency of 1.8 GHz in a 2-path Rayleighchannel; the velocity of the mobile station is assumed to be 25 km/h,and power adjustment is carried out at intervals of 2 ms.

The invention may also be applied in the case of handover in theboundary area between the coverage areas of two base stations. On thebasis of the estimates calculated from measurement results transmittedby the mobile station MS, the base station controller BSC can estimatean optimal point of time for handover in a situation as shown in FIG. 1.By using the method of the invention, the base station controller canpredict the signal behavior in a fading channel, and initiate handoverprocedures more rapidly compared with prior art methods.

In street corner situations, in which the signal propagation environmentmay change extremely rapidly, rapid power adjustment is necessary. Inthe method of the invention, the base station can adjust transmissionpower very rapidly on the basis of the measurement results. FIG. 3illustrates a situation in which a mobile station which is communicatingwith a base station BTS1 comes to a street corner, and to the coveragearea of a base station BTS2. Let us assume that the distance d1 of thebase station BTS1 from the street corner is longer than that d2 of thebase station BTS2. Let us further assume a system in which both basestations transmit by using the same frequency band, which is the case,e.g. in a cellular radio system applying the CDMA-multiple accessmethod.

The mobile station, controlled by base station BTS1, transmits by usinga high transmit power. As the mobile station comes to a street corner,its transmission interferes with base station BTS2 with its hightransmission power. The mobile station must therefore rapidly reduce itstransmission power and change over to the coverage area of BTS2. In acase of this kind, the predictive method of the invention can beemployed in such a way that when approaching the street corner, themobile station notices that the transmission power of base station BTS2gradually increases. As the signal of BTS2 is estimated, its behaviorcan be predicted, and the necessary handover can thus be initiatedrapidly as soon as it becomes possible. Thus, interference caused toother connections in the cell is smaller compared with prior artmethods.

The method can also be used for estimating missing measurement values ofthe mobile station. Measurement results may be destroyed, e.g. due tointerference on the radio path when the results are transmitted to abase station. Missing measurement values can be estimated with recursivecalculations by using equations of the type described above.

Furthermore, the method can be employed for estimating reliability ofthe measurement results received from a mobile station. If a measurementresult received from a mobile station notably deviates from theestimated value, it can be assumed that an error has occurred in datatransmission, and that the transmitted measurement result perhaps is notcorrect.

Thus, in the method of the invention, a model suitable for the cell isfirst selected. In the selection, previously measured power values areused as the basis for the selection of the matrices B and A for theabove shown formulas (b 1), (2) and (5). Once the suitable model hasbeen selected, estimate algorithms may be calculated either forwards orrecursively, depending on whether a signal to be received is predicted,or whether missing measurement values are estimated. Estimates obtainedin this way can further be processed as true power values in handover orpower adjustment algorithms. Kalman filtering and calculationspertaining to it are described in greater detail in Robert Schumway:Applied statistical time series analysis, Prentice Hall, 1988, and P.Strobach: Linear Prediction Theory, Springer Verlag 1990, which areincorporated herein by reference.

In accordance with a preferred embodiment of the invention, when themodel describing dynamic behavior of the signal is formed, one or moreinterfering signals are also taken into account besides the actualsignal. In such a case, it is possible to take possible correlationsbetween signals into account. E.g. in CDMA systems, spreading codes usedfor various connections are not fully independent, and thus correlationoccurs between connections.

FIG. 4 is a block diagram illustrating the structure of the a receiverin which the method of the invention is applied. The receiver comprisesan antenna 30, means 31 for switching the signal to the baseband, means32 for deinterleaving the signal, and means 33 for demodulating thesignal. The receiver further comprises means 34 for controlling theoperation of other blocks and the receiver. The receiver, as will beunderstood, also comprises other components, such as converters andfilters, and depending on the nature of the receiver, also a speechdecoder, but, since these components are not essential to the presentinvention, they are not shown in this illustration.

The receiver in which the method of the invention is applied comprisesmeans 34 for estimating the received signal on the basis of themeasurement results obtained by the mobile station and the base station.Means 34 can be implemented, e.g. by means of a digital signalprocessor.

Although the invention has been described above with reference to theexamples shown in the accompanying drawings, it is obvious that theinvention is not restricted to these examples, but can be modified in avariety of ways within the scope of the inventive concept recited in theattached claims.

We claim:
 1. A method for estimating received power in a cellular radiosystem which has in each cell at least one base station communicatingwith mobile stations within its respective coverage area, in whichsystem mobile stations measure strength of the signal received from arespective base station, and report the measurement results to equipmentof the respective base station, and the respective base station measuresstrength of the signals received from the respective mobile stations,comprising:creating with the aid of the received measurement results, aspace state model describing the dynamic behavior of the respectivesignals for received power on each connection, as said model is beingcreated, taking into account at least one signal interfering with theconnection, calculating an estimate of received power for eachrespective mobile station on the basis of said model, each respectivesaid estimate taking said at least one interfering signal into accountand therefore being non-linear, and utilizing said each said estimateboth for making power adjustments and for making handover decisions. 2.The method as claimed in claim 1, comprising:at each given time t,predicting the power received by each respective mobile station at afollowing moment of time t+ΔT on the basis of said model.
 3. The methodas claimed in claim 1 comprising:constantly updating a state space modelfor each connection in accordance with the measurement of received powercarried out the each respective mobile station.
 4. The method as claimedin claim 1 wherein:in case of a missing result of a measurement carriedout by a respective said mobile station at a given moment of time,calculating an estimate for said missing result on the basis of saidstate space model.
 5. The method as claimed in claim 1 comprising:ateach given time t, calculating the correctness of the measurementcarried out by each respective mobile station by comparing a respectivemeasurement result with a respective estimate calculated on the basis ofsaid model.
 6. The method as claimed in claim 5, wherein:each respectivemobile station carries out measurement for the received power at timeintervals of ΔT.
 7. The method as claimed in claim 5, wherein:incalculating an estimate for the measurement results received from eachrespective mobile station at each given time t, applying the Kalmanfiltering algorithm.
 8. The method as claimed in claim 5, wherein:whencalculating a prediction for the power received by each respectivemobile station at a respective following moment of time, applying theKalman filtering algorithm.
 9. A receiver, for use in a cellular radiosystem having in each cell at least one base station communicating withmobile stations within its respective coverage area, in which systemmobile stations measure the strength of the signal received from arespective base station, and report the measurement results to equipmentof the respective base station, and the base stations measure thestrength of the signals received from the respective mobile stations,the base station equipment having means for recording the measurementresults reported to the respective base station said receivercomprising:model forming means for forming with the aid of the receivedmeasurement results, a model describing the dynamic behavior of therespective signal for the received power on each connection, taking intoaccount at least one signal interfering with the connection, and forcalculating an estimate of received power for each respective connectionon the basis of said model, each respective said estimate taking said atleast one interfering signal into account and therefore beingnon-linear, and means for utilizing each said estimate for poweradjustment, as well as for making handover decisions.
 10. The receiveras claimed in claim 9, comprising:means for predicting at each giventime t, the received power from each respective said connection at afollowing moment of time t+ΔT on the basis of the state space model. 11.The receiver as claimed in claim 9 comprising:means for calculating thecorrectness of the measurement carried out by each respective mobilestation at each given time t by comparing the result with a respectiveestimate calculated on the basis of the state space model.
 12. Thereceiver as claimed in claim 11, functionally incorporated in a mobilestation.
 13. The receiver as claimed in claim 11, functionallyincorporated in a mobile station.